The invention relates to pulse-compression techniques, and more particularly to a pulse-compression technique using a fast Fourier transform circuit in a matched filter.
It is known to raise the average transmitted power of a given radar by expanding the pulse length and simultaneously keeping a constant bandwidth so that the range resolution capability of the radar is not reduced. To achieve this, a long pulse containing some sort of modulation is transmitted. Upon reception, the pulse is compressed to permit separation of adjacent range resolution cells.
One known form of modulation to effect pulse compression is phase modulation in which, within the width of the transmitted pulse, the phase is changed at specified intervals or subpulses. While these phase changes can follow a random sequence, by using certain well-defined sequences known as "Frank codes" it is possible to reduce the level of the sidelobes after processing of the received pulse.
A new uniform-amplitude polyphase code which is amenable to digital processing has been described in the paper "A New Class of Polyphase Pulse Compression Codes and Techniques" by B. L. Lewis and F. F. Kretschmer, Jr., IEEE Transactions on Aerospace and Electronic Systems Vol., AES 17, No. 3 (May 1981) pp. 364-370. This code is similar to the Frank polyphase code in many respects. The similarities include low sidelobe levels, good Doppler tolerance for search radar applications and ease of implementation. The new code is referred to as the P2 code. While the peak sidelobes of the autocorrelation function of the P2 code are the same as the Frank code for the zero Doppler case, the mean square sidelobes of the P2 code are slightly less (approximately 1 dB for a 16-element code decreasing with the pulse compression ratio).
The significant advantage of the P2 code over the Frank code is that it is more tolerant of receiver bandlimiting prior to pulse compression. Such bandlimiting will be encountered in radars employing digital signal processing. This bandlimiting is required to avoid out-of-band noise foldover caused by sampling in conversion to digital format.
A disadvantage of prior art digital pulse expander-compressors which use the P2 code is that they require a large amount of electronic hardware before significant results can be obtained and ways of reducing the amount of hardware have been sought, for example by taking advantage of the symmetry of the phases, which allows a savings of (1/2)N.sup.2 phase shifters or complex multipliers, where N.sup.2 equals the pulse compression ratio or number of code elements. A discussion of prior art digital expander-compressors using the P2 code may be had by referring to U.S. patent application Ser. No. 143,399 by B. L. Lewis entitled "Pulse Compression System".